1. Field of the Invention
The invention relates to an apparatus for recording a digital video information signal and a corresponding trick play signal onto a record carrier, said digital video information signal being meant for a reproduction from said record carrier at a nominal play reproduction speed, said trick play signal being meant for a reproduction from said record carrier at a trick play speed m times said nominal reproduction speed, m being an integer larger than 1. The invention further relates to a method of recording such trick play signal and to a record carrier.
2. Description of the Related Art
An apparatus as defined in the opening paragraph is known from published International Patent Application WO 95/28061 (PHN 14832), which corresponds to U.S. Pat. No. 5,751,889.
Bit-rate reduction of digital television signals has been an area of interest for more than three decades. This has resulted in an ISO standard for the coded representation of video and associated audio data. The first MPEG publication dated April 1992, which resulted in the introduction of MPEG-1. This system was designed to reduce the bit-rate down to 1.5 Mbits/s. To increase the video quality and still use low bit-rates, MPEG-2 has been developed. This technology has been selected to be used in Digital Video Broadcasting (DVB). DVB has the potential to transmit studio quality video at an acceptable low bit-rate. This enables the customer to receive studio quality signals at his own place. In order to enable the customer to store a DVB program with studio quality, a digital video recorder is required. For two years, Digital Video (DV) recorders and digital camcorders have been available for the consumer market. Both systems make use of dedicated video bit-rate reduction technology which is not compatible with the compression technology that is used in MPEG-2. In order to store a selected DVB program and maintaining the high quality, a transparent recorder is required. Although a standard has been defined to store MPEG on a DV recorder, up to now, no equipment has been produced that support this standard. A popular consumer audio-visual storage device is the VHS-based video recorder. This system is capable of storing and retrieving analog audio-visual programs. To enable storage of digital television programs, a digital extension is currently developed to enable the consumer to store and retrieve DVB programs. The currently developed standard describes the record and playback aspect of this system. Not yet included in this standard is how to perform trick play. Trick play, based on the track select system for D-VHS MPEG-2 STD mode format, will be described herein.
Since June, 1996, a new standard has been released, D-VHS MPEG2 STD mode format. This digital version of the VHS recorder family is capable of recording DVB signals at up to 13.8 Mbits/s. The standard, as it is currently available, only describes the record- and playback process. Visual search, better know as trick play, has not yet been defined.
It is an object of the invention to provide proposals for realizing trick play in general, and more specifically, for realizing trick play for D-VHS MPEG2 STD mode format.
In accordance with the invention, the apparatus for recording a digital video information signal on a record carrier comprises:
input means for receiving the digital video information signal,
trick play signal generating means for generating a trick play signal from said digital video information signal, so as to enable a trick play reproduction at a speed m times the nominal reproduction speed, where m is an integer larger than 1,
merging means for merging said digital video information signal and said trick play signal into a composite information signal,
writing means for writing said composite information signal in a track on said record carrier, said trick play signal generating means being adapted to
(a) retrieve intra-encoded pictures from said digital video information signal,
(b) generate inter-encoded pictures,
(c) merge said intra-encoded pictures and said inter-encoded pictures, so as to obtain a trick play signal comprising subsequent groups of pictures, comprising an intra-encoded picture, followed by a number n of said generated inter-encoded pictures, where n is an integer larger than 0, the generated inter-encoded pictures being such that, upon reproduction at said trick play speed, an inter-encoded picture following an intra-encoded picture results, upon decoding, in a repeated presentation of a picture obtained from decoding said intra-encoded picture.
The intra-encoded pictures could be in the form of intraframe-encoded pictures, or in the form of intrafield-encoded pictures. Further, inter-encoded pictures could be in the form of interframe-encoded pictures, or in the form of interfield-encoded pictures. In the following description, it will be assumed that the intra-encoded pictures are in the form of intraframe-encoded pictures and that the inter-encoded pictures are in the form of interframe-encoded pictures.
The invention is based on the following recognition. The generation of the trick play information signal is based on the retrieval of intraframe-encoded pictures, such as I-pictures in accordance with the MPEG format, from the normal play information signal. Simply using intraframe-encoded pictures (I-pictures) with an acceptable refresh rate in the trick play information signal is not possible, as the bandwidth available for the transmission of those intraframe-encoded pictures in the trick play information signal is too small. In order to overcome this, so-called xe2x80x98emptyxe2x80x99 interframe-encoded pictures, such as xe2x80x98emptyxe2x80x99 P-pictures, and/or xe2x80x98emptyxe2x80x99 B pictures, are generated and the datastream of the trick play information signal is built up of GOPs, each GOP comprising an intraframe-encoded picture (I-picture) and one or more of those xe2x80x98empty interframe-encoded picturesxe2x80x99 (e.g., empty P-pictures). Such xe2x80x98emptyxe2x80x99 interframe-encoded pictures result, upon decoding in the repeated presentation of the intraframe-encoded picture that preceded the one or more xe2x80x98emptyxe2x80x99 interframe-encoded pictures. As the xe2x80x98emptyxe2x80x99 interframe-encoded pictures require a relatively low bit content, it has now become possible to realize a trick play information signal that realizes upon reproduction and subsequent decoding in a trick play reproduction mode in a reproduced video signal with pictures having a sufficient frame rate. Further, such a GOP structure (as an example, an IPP . . . GOP structure), a sufficient refresh rate can be obtained.
A further aspect of the invention is that GOPs are generated for the trick play information signal with a constant bit-cost. This has the advantage that a trick play GOP can be re-used for higher trick play video speeds.
Another aspect of the invention lies in the generation of the intraframe-encoded pictures for the trick play information signal from the information comprised in the normal play information signal. More specifically, an intraframe-encoded picture comprises a DC coefficient and a number of AC coefficients. The DC coefficients of the sub-pictures of an intraframe-encoded picture in the normal play information signal are retrieved therefrom and used in the trick play information signal as the DC coefficients of the corresponding sub-pictures of an intraframe-encoded picture in said trick play information signal. Further, from a sub-picture of the same intraframe-encoded picture of the normal play information signal, a restricted number of AC coefficients from that picture is retrieved to form the AC coefficients for the corresponding sub-picture of the intraframe-encoded picture in the trick play information signal to be generated. This results in a reduction of the number of bits in the intraframe-encoded picture generated, compared to the intraframe-encoded picture of the normal play information signal. The number of AC coefficients selected for a sub-picture of a specific intraframe-encoded picture of the trick play signal depends on the difference between two DC coefficients, those DC coefficients being the DC coefficient of the said sub-picture and the previous sub-picture of the specific intraframe-encoded picture in the trick play signal.
The signal processing aspects of low-end trick play will be described. Low-end trick play means a trick play signal processing algorithm that re-uses pre-encoded MPEG video material to create video trick play.
The video trick play signal processing in accordance with the invention results in a low hardware complexity. For the video trick play signal processing, a wide variety of architectures can be applied. For all these architectures, two main parameters can be used to control the final quality. The first parameter is the spatial resolution of the MPEG-encoded picture. The second parameter is the temporal refresh rate at which the viewer perceives the decoded pictures. The low-end video trick play signal processing algorithm, will be based on MPEG pre-encoded video information, as it is available in case of a DVB broadcast. Beside a wide variety of trick play signal processing algorithms, there is also some variety in the system used to implement trick play. The trick play system used in this report is based on track select.